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Q: What’s a car rental company’s responsibility to have a vehicle available for you when it sends you a confirmation? I ask because we recently rented a car from National Car Rental in Mexico, and they ran out of cars.

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When we arrived at the rental counter in Guadalajara, a representative told us that other renters did not return their cars on time. He said there would be no cars for several days and would not help us find another car.

We eventually got a rental from another company, but it took us an additional five hours, and we were very upset by the experience. Although our first email to National was acknowledged with a form response, we haven’t heard anything from the company in two months. I thought rental companies had to either honor their reservation or find a car from another rental company. Isn’t that what National should have done? — David Nicholson, Halifax, Nova Scotia

A: National should have found you another car — even if it was from a competitor. That’s the policy of most major rental companies, including National. The Guadalajara office goofed.

Why do car rental companies confirm reservations when there’s a chance they won’t have a car? One reason is that customers can cancel their reservation without penalty, or just not show up at all. In fact, as many as a third of the people with reservations are no-shows at some rental locations, according to informal estimates I’ve heard. (It’s usually between 15 and 25 percent.)

That means car rental companies have to factor in the possibility that lots of the people who reserve a car won’t show up when they’re managing their fleet. And that can be a real guessing game.

You checked in on a day when National guessed wrong. It had run out of cars. The rental employee must have known National’s policy, but instead chose to let you fend for yourself in a foreign country. Something tells me he won’t be getting his Christmas card from the Mexico Tourism Board this year.

You shouldn’t have taken a frontline employee’s “no” for an answer. Instead, you should have asked for a manager, and if one wasn’t available, you should have called National’s reservation number to report this. National could have helped you find another car quickly.

Car rental employees often treat foreign tourists differently from natives, and this seems to be a case-in-point. It could have been worse. I’ve heard horror stories of out-of-country visitors being forced to buy unnecessary insurance or talked into expensive upgrades. My best advice would be to be on your guard when you rent overseas. The moment you open your passport, you become a walking dollar sign.

I contacted National on your behalf, and it refunded you $685, the cost of your car rental in Mexico, and sent you a coupon for three rental days as an apology.

Christopher Elliott is the ombudsman for National Geographic Traveler magazine and the host of “What You Get For The Money: Vacations” on the Fine Living Network. E-mail him at celliott@ngs.org.